This invention pertains to bits used for boring holes in the earth and more particularly to such bits use in rotary percussion drilling with dual conduit drill stem. In such a system air or other drilling fluid flows down the outer conduit of the drill stem, through down fluid passages in the bit body, across the bottom of the bit and hole being bored, and back up the inner conduit of the drill pipe, carrying with it the detritus. The bit is both rotated and vibrated up and down by means connected to the upper end of the drill stem. The system is especially useful for drilling holes in permafrost to receive foundation members, e.g. to support an elevated fluid transmission pipe line. In such use the drill stem may comprise two lengths of drill pipe, one thirty-five feet long and the other fifteen feet long and the drill rig will be adapted to push the drill stem down as well as to rotate and vibrate it.
Conventional bits used in rotary percussion drilling wear out rapidly, whereupon it is necessary to replace the whole bit.